ALBERT

Description: The Centennial U deposit is situated in the south-central Athabasca Basin (Canada) and straddles the unconformity between early Paleoproterozoic to Archean metasedimentary and metavolcanic rocks and granitoids, and the clastic sediments of the Paleoproterozoic Athabasca Group. Although it has most characteristics of an unconformity-related uranium deposit, the Centennial deposit is atypical in that it is not directly associated with an electromagnetic conductor (there is a paucity of graphite in the basement) or with a major reverse fault zone; it is distal from a major fluid conduit (ca. 300 to 400 m from the Dufferin Lake Fault), has low Ni, Cu, Co, Zn, and Pb contents, and contains an unusually large amount (up to 5%) of secondary uranyl minerals. Additionally, a network of diabase dikes and sills is observed at Centennial, seemingly intruding the main U mineralization of massive uraninite, based on the relatively sharp contacts between the diabase dike and the high-grade U mineralization.The pre-U alteration assemblage at Centennial includes kaolinite, illite, and sudoite, which have been formed by fluids with isotopic and chemical compositions that are comparable with those from other sandstone-hosted unconformity-type U deposits in the Athabasca Basin. Pre-U illite-related fluids have d18O of ca. 3‰ and dD of ca. -40‰, whereas pre-U chlorite-related fluids have d18O between 1.7 and 4.3‰ and dD between -18 and 1‰.Laser ablation-inductively coupled plasma-mass spectrometry (LA-ICP-MS) U/Pb dating of the various U phases indicates that initial mineralization, represented by disseminated uraninite found directly to the north-northeast of the Centennial deposit, occurred at ca. 1.6 Ga. The main U mineralization, represented by massive and strongly altered uraninite, followed at an unknown time. A minor (

Description: The Centennial unconformity-related uranium deposit represents the first significant uranium mineralization along the Snowbird tectonic zone in the south-central part of the Athabasca Basin. The deposit is associated with a steeply WNW-dipping contact between the Virgin Schist Group and mylonitic granite of uncertain age. Virgin Schist Group rocks in the deposit area include phyllites, impure quartzite, and quartzite. Medium- to coarse-grained microcline "augen" porphyroclasts characterize the granite. The quartzite, being resistant to weathering, forms a paleotopographic high flanked by a paleotalus which formed prior to Athabasca Group sedimentation. Diabase, petrographically and geochemically similar to the 1.27 Ga Mackenzie dikes, intrudes the mineralized trend along brittle structures in the basement rocks and Athabasca Group. A detailed paragenetic study of the deposit area reveals a protracted history that is related to the episodic reactivation of brittle structures and associated fluid movement along this significant structural corridor. Retrograde metamorphism converted biotite to chamosite at temperatures between 335° to 350°C. Weathering caused the breakdown of microcline and the dissolution of quartz prior to erosion and deposition of the Athabasca Group, which started at about 1750 Ma. Compaction and diagenesis resulted in quartz overgrowths and development of a diagenetic clay assemblage of illite and sudoite at temperatures between 150° and 200°C, which was broadly synchronous with primary mineralization. Clinochlore, euhedral quartz, carbonate, and pyrite was developed after the intrusion of the diabase under reducing conditions, primarily at temperatures between 270° to 320°C. Uranium remobilization and alteration of uraninite to coffinite appears to have taken place after the emplacement of the diabase dikes. Kaolinite and uranophane form last in the paragenetic sequence and probably form at relatively low temperatures.